High-temperature apparatus members, such as industrial gas turbine blades, jet engines, or the like, for example, are subject to a fluid temperature in excess of 1300° C. Those members, which are made of a metal material, are susceptible to damage due primarily to high-temperature oxidation. For the purpose of making the members resistant to heat, it has heretofore been the general practice to apply a coating to surfaces of the members according to one of the following processes:
(1) Thermal Barrier Coating (TBC):
Thermal barrier coating (TBC) is a laminated assembly of a ceramics layer called a topcoat and a corrosion-resistant alloy layer called an undercoat (or a bond coat), which are successively deposited on a surface of a metal base (member). The topcoat is generally made of ZrO2 having a small coefficient of thermal conductivity for mainly lowering the surface temperature of the metal base to about 1000° C. or lower. The undercoat is, on the other hand, generally made of an alloy (normally referred to as MCrAlY) containing several to several tens % of Al in order to make it resistant to oxidation.
In recent years, fluid temperatures tend to be higher for higher electric power generating efficiency, and accordingly the surface temperature of the undercoat also tends to be higher. This tendency results in a serious problem in that a thick oxide film grows in the interface between the undercoat and the topcoat, causing the topcoat to peel off and at the same time causing Al to be diffused from MCrAlY into the metal base thereby to reduce the strength of the metal base. Even at conventional temperatures, jet engine turbine blades, for example, are said to have a service life of a half year even if the thermal barrier coating is applied to their surface. Consequently, there has been a strong demand for the development of a technology for extending the service life of these members. It has been said that one of the major causes of the degradation of the TBC system is a mutual diffusion of alloy components between the undercoat and the metal base.
Furthermore, the TBC system requires a topcoat having a thickness of several hundreds μm and cooling air for increasing the effect of temperature reduction. Therefore, the TBC is generally not suitable for use in small regions and regions where cooling air is not available.
(2) Al (or Cr, Si) Diffusion Process:
Members (metal bases), which need to have oxidation resistance and high-temperature corrosion resistance at 1000° C. or lower, are often treated with Al, Cr, or Si by the diffusion process. It is known that the oxides of these elements have a small ion diffusion capability and hence members whose surface is coated with these oxides are less susceptible to high-temperature oxidation and high-temperature corrosion. To form these oxides, therefore, a surface of a member is coated with an alloy film containing several tens % of one of these elements. One typical coating process is known as a diffusion process. An alloy film (coating film) formed by this process is highly adhesive to a member (metal base) as it forms a diffused layer, and is applicable to components having complex shapes and to small parts.
However, as with the above-described TBC system, when the alloy film thus formed is used at a high temperature for a long period of time, a mutual diffusion of alloy components occurs between the alloy film and the metal base, reducing the concentration of Al (or Cr, Si) in the alloy film to the extent that sound corrosion-resistant oxides are no longer available.
(3) Ni—Cr or MCrAlY Spraying:
It is also generally customary to spray Ni—Cr or MCrAlY onto a surface of a metal base to form an alloy film thereon. The spraying process is advantageous in that the composition of the alloy film can freely be selected. However, since the alloy film is a porous film, it is generally difficult to form a good-quality film as a high-temperature-resistant, corrosion-resistant coating layer. Furthermore, the spraying process is defective in that the use of a spray gun puts a limitation on the shapes of members to which the spraying process is applicable, and it is difficult to form thin films having a thickness of about 10 μm or smaller. Though the sprayed alloy film remains effective in short-term usage, it tends to reduce the corrosion resistance of the metal base (member) when used at high temperatures for a long period of time for the same reasons as described above in (2).
(4) Deposition (PVD), Particularly Electron Beam Deposition (EB-PVD):
In recent years, attention has been paid to EB-PVD as a process of forming TBC. This is because EB-PVD is capable of forming a dense, thick (several hundreds μm), and uniform metal film unlike PVD that finds it difficult to form thick metal films.
However, it is generally difficult to apply EB-PVD to parts having small clearances though it is possible to form a metal film on a curved surface by rotating the metal base. EB-PVD is also highly costly to perform. As with above-described (1) through (3), the degradation of an alloy film formed by EB-PVD is unavoidable due to a mutual alloy diffusion between the alloy film and the metal base when the member is used for a long period of time or at an ultrahigh temperature.
(5) Pt Electroplating+Al Diffusion:
In recent years, it has been known that a plated film of Pt is formed on a surface of a metal base (member) by electroplating and thereafter Al is diffused into the plated film to produce an oxidation-resistant coating on a jet engine turbine blade, for example. Specifically, Pt is added to nickel aluminide (i-NiAl) that is widely used as a corrosion-resistant layer material for thereby stabilizing the layer to keep the alloy film (coating layer) in sound conditions for a long period of time.
(6) TBC System with a Re-Added Undercoat:
There has been proposed a TBC system including an undercoat to which 12 weight % (several mol %) or less of Re is added (see, for example, Japanese laid-open patent publication No. H11-61439). A TBC system containing 35 to 60 weight % (about 15 to 30 mol %) of Re has also been proposed (see, for example, PC(WO) No. 2000-511236). However, no detailed description has been given as to the role of Re, and the effect of Re is uncertain.
(7) Diffusion Barrier of Re—Cr Alloy:
A common problem of the technologies described above in (1) through (6) is that when the member has been used at a high temperature of about 1000° C. or higher or for a long period of time at a temperature of 1000° C. or lower, the concentration of Cr, Al, or Si in a corrosion-resistant oxide film coating layer of, e.g., Al2O3, Cr2O3, or SiO2 is lowered due to a mutual alloy diffusion between the coating layer (alloy film) and the metal base, making the coating layer less resistant to corrosion. When Pt-added β-Ni(Pt)Al has been used at a high temperature of 1000° C. or higher or for a long period of time at a temperature of 1000° C. or lower, since Pt has a low melting point of about 1770° C., it is expected that Pt is diffused into the metal base, and the coating layer becomes less resistant to corrosion.
The inventors have proposed a Re alloy film for use as a diffusion barrier for preventing a mutual diffusion between a coating layer and a metal base (see Japanese laid-open patent publication No. 2001-323332). The inventors have also proposed an Re—Cr alloy film (see International Publication No. 03/038150), an Re—Cr—Ni alloy film (see International Publication No. 03/038151), and an Re—(Cr,Mo,W)—(Ni,Co,Fe) alloy film (see International Publication No. 03/0381512) as alloy film compositions having an excellent diffusion prevention capability. These diffusion barrier alloy films mainly have an Re—Cr alloy σ phase as a basic composition, and may have their composition optimized for the base, the application, and the temperature range in which they are to be used.